Primary emission controlled tube



July 24, 1956 G. D. O'NEILL PRIMARY EMISSION CONTROLLED TUBE Filed Nov.25, 1948 INVEN TOR. Geo/"949D Oweill United States Patent Ofice2,756,360 Patented July 24, 1956 PRIMARY EMISSION CONTROLLED TUBE GeorgeD. ONeill, Manhasset, N. Y., assignor to Sylvania Electric ProductsInc., a corporation of Massachusetts Application November 23, 1948,Serial No. 61,703

11 Claims. (Cl. 313-103) The present invention relates to electrontubes, more particularly to the type of positive-grid tubes including asecondary emitter electrode. Tubes of this class are disclosed in U. S.Patents Nos. 2,707,771 of May 3, 1955; 2,679,591 of May 25, 1954; and2,677,057 of April 27, 1954, all of which are assigned to the assigneeof the present invention.

Quick-starting requirements, and conservation of energy needed to heat acathode encourages the use of directly heated filamentary cathodes,coated with electron-emissive material. Difierences among positive-gridtubes as to the length of cathode wire that is coated lead to variationsin the space-charge-limited currents among those tubes. This in turnleads to the possibility of excess drain imposed by the tube on a powersupply of limited capacity, and to possibility of excess internal powerdissipation and consequent deleterious heating of the tube elements;conversely, the current may be lower than desired with resultant loweredperformance. Accordingly, an object of this invention is to provide anovel positive-grid tube, such that there will be improved uniformity ofspace-charge-limited current among tubes of one type.

In positive-grid tubes, there is a tendency of the electron stream toapproach not only the active portions of the grid, but additionally themetallic connections and supports for the positive grid outside theactive region, thus reducing tube efficiency. A further object of theinvention is to improve tube efliciency, more specifically toconcentrate the electron stream in the region where it can beefficiently utilized and will efiect the intended functions.

An additional object of the invention is to reduce the current densityin the cathode surface without changing the specified value of cathodecurrent.

In dynatrons or positive-grid tubes having a secondaryemitting electrodeor dynode as described in the copending applications mentioned abovethere is the possibility that secondary electrons will penetrate thegrid and aifect the primary current under some conditions. This tends torender the tube unstable and, accordingly, a further object is to avoidthe adverse effects of such secondaryelectron penetration.

The foregoing purposes are achieved by including an apertured controlelectrode between the cathode and the positive grid which is of suchform as to confine the electron stream to the active portion of thegrid, thus improving efiiciency. This control electrode may be operatedat cathode potential or it may be negative with respect to cathode, thusfixing the value of the cathode current and permitting the use ofcathodes of larger extent without increasing the current. The resultingcurrent-density in the cathode surface is consequently reduced.

Comparatively wide variations should be expected in the extent ofelectron-emissive material on the cathode due to the intrinsic nature ofits processing, and this would cause correspondingly wide variations incathode current. The control electrode, which is a centrally apertureddisc in the illustrative embodiment, thus regulates thespacecharge-limited current. Unlike the cathode, the control electrodecan be made and positioned with a high degree of accuracy with resultinguniformity of current characteristics.

This electrode controls and stabilizes not only the primary electroncurrent, but in dynatron tubes this electrode tends to minimize theeffect on the primary emission that may otherwise be caused shouldsecondary electrons from the dynode penetrate the grid.

The electrode between cathode and the positive grid serves the furtherpurpose of enabling measurement of ionization in the cathode-gridregion, which is a valuable check on the quality of the tube not readilydeterminable in tubes of the forms in the aforementioned copendingapplications. When operating the tube, with the electrode slightlynegative as it will be when connected to the negative terminal of itsbattery-heated filament, the electrode tends to collect positive ionswhich would otherwise contribute to the noise level of the tube as isespecially true in the case of filamentary cathodes that are of coatedlimited extent.

The invention will be better appreciated from the following detaileddisclosure of an illustrative embodiment which is shown in theaccompanying drawings:

'Fig. l is a fragmentary longitudinal sectional view of an illustrativetube embodying the invention;

Fig. 2 is a transverse sectional view of the tube in Fig. 1 along theline 22; and

Fig. 3 is a fragmentary sectional view along the line 3--3 in Fig. 2.

Referring now to the drawings, a vacuum tube is shown having an upperportion 10 of its envelope wall formed of glass and the lower portion 12of its wall formed of metal. The bottom of the envelope includes a metalring 14 joined to wall portion 12, and glass plug 16 which incorporatessealing tip 18 and provides mutually insulated seals for several of theleads emerging from electrodes.

Internally, filamentary cathode 20, consisting of two parts 20a and 20b,is stretched taut across an aperture 22 in mica wafer 24 that is carriedby four supporting rods 26 secured to ring 14. The terminals of cathode20 are connected to leads 23 and 30 which are sealed through glass plug16. The terminals of the cathode 20 are connected to the leads 28, 30through L-shaped conductive supports 28a, 30a. Coil spring 32 engagesthe midpoint of the cathode to maintain its tension. Portions 20a and20b of the cathode which extend across aperture 22 in the bottom micaare coated with electron-emissive material. In the portions of thefilamentary cathode that are directly in contact with mica 24 there is aslight cooling, in comparison to the coated cathode portions aboveaperture 22. However the entire filamentary cathode passes heatingcurrent, and all its coated portions contribute to the electron stream.

In addition to filamentary cathode 20, included in the tubes describedin the copending patent applications mentioned above, the furtherelectrodes include positive grid 34 and dynode 36, which is asecondary-emissive electrode. The secondary-emission ratio of the dynodeis purposely enhanced by application of such materials as magnesiumoxide or caesium oxide or other comparable materials known in the art.The term secondary-emissive electrode is not intended to includeelectrodes that may emit some small number of secondary electrons but,rather, electrodes having surface characteristics adapting them to emitsecondary electrons copiously upon bornbardment by incident electrons.Grid 34, of wire mesh in the illustration, is convex toward the cathodeand is secured to the annular internal extension 38 of metal envelopeportion 12. The extension 38 serves as a terminal for the application ofappropriate biasing potentials to the grid 34. The grid is of arefractory metal such as tungsten so as to withstand the heating usuallycharacteristic' of positive-grid operation. .Dynode 36 is sealed throughthe upper envelope portion at a point not shown. In the tube thus fardescribed, with appropriate positive potentials applied to grid 34 anddynode. 36, the cathode current will reach a value limited by spacecharge in the region of the cathode, and'will be approximatelyproportional tothe length of the coated sections 2011 and 20b of thefilament, to the three-halves power of the voltage at the grid, and inapproximate inverse proportion to the distance between the grid and thefilament. In view of the direct relation between the coated length andcurrent, and also in view of the fact that difiiculty is experienced inmaintaining accurate dimensioning of the coated length of filament,intolerably wide variations in space-charge-limited current are normallyto be expected. Furthermore, while the active portion of the grid, themesh in this specific tube, is closest to the cathode and thereforedraws a large share of the cathode or primary electron emission, thegrid support 38 can be expected to draw a share of the cathode currentand thereby reduce the "tube efficiency and increase the burden on thepower supply. Especially in portable applications, the power supply isof limited capacity. In passing let it be understood that the term gridis used in the broad sense to mean any electron-permeable electrode,unless specifically indicated otherwise.

In order to improve the tube in the face of the foregoing conditions, anadditional electrode 40 having an aperture 40a has been added, theelectrode being in the form of a cupped disc larger in outer diameterbut advantageously smaller in inner diameter than the aperture in gridsupport 38. The aperture 40a can be made larger than the active portionof the grid but then it should be operated at negative bias if it is toshield the grid support that is inactive. Electrode 40 is supported byand confined between a pair of centrally apertured micas 42 and 44 whichare conveniently secured to rods 26, and has its own lead 46 sealedthrough glass plug 16. Electrode 40 thus has an externally-availableterminal for application of any suitable potential or for'connection asat 48, Fig. 3,'to the cathode itself where desired. Alternatively,electrode 40 could be connected to the cathode internally of the tube;but it would then not be available for biasing when desired, or formeasurement of the residual gas in the tube.

Electrode 40 constitutes a mask, or an electrostatic shield whichintercepts electrostatic lines between the cathode and the inactiveportions of the grid structure that are close enough to the cathode todraw appreciable current; Electrode 40 is conveniently positioned aboutmidway between cathode 20 and positive grid 36, and has the affect ofclosely regulating the value of cathode current despite increases abovespecifications in the coated length of filamentary cathode. In theimproved tube, the length of cathode coating, which is ditficult toduplicate accurately, is much greater than in the tube previouslydisclosed so thatit is not a controlling factor in the total cathodecurrent; this function is now taken over by electrode 40, the apertureof which is more readily and accurately duplicated from tube to tube.Improved uniformity of cathode current is thus achieved. Due to thegreater coated length, the cathode current is reduced for longer tubelife and better uniformity among tubes.

What is claimed is:

1. An electron-discharge tube of the high-vacuum positive-grid typecomprising a secondary-emissive anode, a substantially planar gridconstructed to withstand the heating of positive-grid operation, and afilamentary cathode, all mounted in an envelope in the order named insubstantially parallel planes, an annular metal support and terminaljoined to said grid and extending radially there-- from, a metalelectrode interposed between said filamentary cathode and said grid andhaving an aperture therein substantially smaller than saidpositive-grid, and

means for applying a biasing potential to said metal electrode.

2. An electron-discharge device comprising a planar grid, an annularmetallic support and terminal centrally apertured and joined at theapertured edge to said grid, a filamentary cathode extending in a planegenerally parallel to said grid, a disc-like electrode having anaperture therein and interposed between said filamentary cathode andsaid grid, said aperture being smaller than the extreme transverselimits of said filamentary cathode and said grid, and means including aterminal externally of said device for applying a biasing potential tosaid disc-like electrode.

3. A high-vacuum positive-grid electron-discharge tube comprising atensioned filamentary cathode having a length coated withelectron-emissive material, an insulating wafer supporting saidfilamentary cathode and locating said coated portion in a plane, a gridof refractory metal directly opposite said cathode, and a centrallyapertured disc interposed between said cathode and said grid, theperiphery of said disc being gripped between additional insulatingwafers assembled against said cathode-supporting wafer, the lateralextent of the coated portions of said filamentary cathode being greaterthan the aperture in said disc.

4. An electron-discharge tube comprising a filamentary cathode havingportions coated with electron-emissive material, a generally parallelgrid of refractory wire supported opposite said cathode, and anelectrode interposed between said cathode and said grid, said electrodehaving an aperture therein smaller in' lateral dimension than thelateral extent of both said cathode and said grid, and means forapplying a biasing potential to said electrode.

5. An electron-discharge tube comprising a'filamentary cathode havingportions coated with electron-emissive material, a refractory gridelectrode supported opposite and generally parallel to said cathode, afurther electrode on the side of said refractory electrode opposite thecathode side thereof, and a shielding electrode between said cathode'andsaid grid electrode having an aperture therein smaller in lateraldimension than the extent of said coated portion of said filamentarycathode, and means including a terminal externally of said tube forapplying a biasing potential to said shielding electrode.

6. An electron-discharge device comprising a cathode, a grid ofrefractory metal, and a secondary-emitting electrode mounted in anenvelope in the order named, the improvement consisting of a furtherelectrode between said cathode and said grid having a central aperturefor primary electron passage shielding said cathode from secondaryelectrons that may penetrate said grid, and means connected to saidfurther electrode and including a terminal externally of said envelopefor applying a biasing potential to said further electrode.

7. An electron discharge tube including an evacuated envelope, afilamentary cathode having plural strands disposed in a plane andprovided with supporting and tensioning means to maintain the strands inthat plane, a grid, and a secondary emissive electrode, said grid andelectrode having active portions in closely adjacent planessubstantially parallel to that of the cathode, and a shield interposedbetween the cathode and the grid, said shield having a centered apertureof smaller extent than the transverse extent of said strands.

8. An electron discharge tube including an evacuated envelope, a cathodesupport, a filamentary cathode having portions coated with electronemissive material, said portions being disposed in a plane defined bysaid cathode support 'tensioning means maintaining said filament in saidplane, an electrode having an active surface substantially parallel tothe plane of the cathode, and a shielding electrode between the activeportions of the cathode and said parallel electrode, said shieldingelectrode having an aperture smaller in lateral dimension than theextent of said coated cathode portion.

9. An electrondischarge tube includingan evacuated envelope, afilamentary cathode having plural strands disposed in a plane, supportand tensioning means operatively connected to said filamentary cathodeto maintain said strands in said plane, a grid, a secondary-emissiveelectrode, said grid and secondary-emissive electrode having activeportions in closely adjacent planes substantially parallel to the planeof said filamentary cathode, a shielding electrode interposed betweensaid cathode and said gn'd, said shielding electrode having a centralaperture of smaller extent than the transverse extent of said strands ofsaid filamentary cathode, and means for applying a bias potential tosaid shielding electrode.

10. An electron-discharge tube of the high-vacuum positive-grid typecomprising a secondary-emissive anode, a substantially planar gridconstructed to withstand the heating of positive-grid operation, and afilamentary cathode, all mounted in an envelope in the order named insubstantially parallel planes, an annular metal support and terminaljoined to said grid and extending radially therefrom. a metal electrodeinterposed between said filamentary cathode and said grid and having anaperture therein, and means for applying a biasing potential to saidmetal electrode.

11. An electron-discharge device comprising a planar grid, a filamentarycathode extending in a plane generally parallel to said grid, adisc-like electrode having an aperture therein and interposed betweensaid filamentary cathode and said grid, said aperture being smaller thanthe extreme transverse limits of said filamentary cathode, and meansincluding a terminal externally of said device for applying a biasingpotential to said disc-like electrode.

References Cited in the file of this patent UNITED STATES PATENTS2,091,443 Heintz Aug. 31, 1937 2,384,087 Goodrich Sept. 4, 19452,407,607 Cairns Sept. 10, 1946 2,460,141 McArthur Jan. 25, 19492,462,921 Taylor Mar. 1, 1949 2,517,726 Skcllett Aug. 8, 1950

1. AN ELECTRON-DISTANCE TUBE OF THE HIGH-VACUUM POSITIVE-GIRD TYPECOMPRISING A SECONDARY-EMISSIVE ANODE, A SUBSTANTIALLY PLANAR GRIDCONSTRUCTED TO WITHSTAND THE HEATING OF POSITIVE-GRID OPERATION, AND AFILAMENTARY CATHODE, ALL MOUNTED IN AN ENVELOPE IN THE ORDER NAMED INSUBSTANTIALLY PARALLEL PLANES, AN ANNULAR METAL SUPPORT TERMINAL JOINEDTO SAID GRID AND EXTENDING RADIALLY THEREFROM, A METAL ELECTRODEINTERPOSED BETWEEN SAID FILAMENTARY CATHODE AND SAID GRID AND HAVING ANAPERTURE THEREIN SUBSTANTIALLY SMALLER THAN SAID POSITIVE-GRID, ANDMEANS FOR APPLYING A BIASING POTENTIAL TO SAID METAL ELECTRODE.